The need to reduce the cost and size of electronic equipment has created a need for smaller signal filtering elements. Thin-Film Bulk Acoustic Resonators (FBARs) and Stacked Thin-Film Bulk Wave Acoustic Resonators (SBARs) represent one class of filter elements with potential for meeting these needs. These filters can collectively be referred to as FBARs. An FBAR is constructed from acoustic resonators using bulk longitudinal acoustic waves in thin-film piezoelectric (PZ) material. Typically, an FBAR includes a layer of PZ material sandwiched between two metal electrodes. The combination PZ material and electrodes are suspended in air by supporting the combination around its perimeter or placing the combination over an acoustical mirror.
When an electrical field is created between the two electrodes, the PZ material converts some of the electrical energy into mechanical energy in the form of acoustic waves. The acoustic waves propagate in the same direction as the electric field and reflect off the electrode-air or electrode-acoustical mirror interface at some frequency, including at a resonance frequency. At the resonance frequency, the device can be used as an electronic resonator. Multiple FBARs can be combined such that each are elements in RF filters.
There is a need to synthesize FBAR filters in certain circuit applications. Currently, it is typical for electrical coupling to be used in synthesizing FBAR filters. Electrical coupling of FBAR filters greatly limits the ability to achieve various filter types and performances in wireless communication applications such as broadband attenuation, impedance transformation, and single-in-differential-out conversion. Although some surface acoustic wave (SAW) filters have been synthesized using acoustic coupling, such acoustic coupling has not been effective in synthesizing FBAR filters.
For these and other reasons, a need exists for the present invention.